I mean the easy answers so far would be either Meracydia or the New World.

Also forgot to add that the special boss shown in the recent LL that would be playable at this years fanfest on record have been dungeon bosses for next expansions.

Gilded Araya being a fight against the Queen of Eidolons from FFIV being Asura, makes me think perhaps New World (because city of gold)